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International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
Volume 4 Issue 7, July 2015
www.ijsr.net Licensed Under Creative Commons Attribution CC BY
Xeroradiography, Digital Radiography and
Computerized Tomography: A Systemic Review
Mahadev Patel1, Shrikant Patel
2
1Assistant Professor, Department of Medicine, Gujarat Adani Institute of Medical Science
2 Senior Lecturer, Department of Oral & Maxillofacial Pathology, Pacific Dental College & Hospital, Debari, Udaipur
Abstract: Advanced diagnostic technologies are increasingly playing a more vital role in both data collection and assessment
capabilities, and the utilization of the information obtained. Diagnostic modalities available to clinicians today expand greatly on the
foundation of a comprehensive visual assessment, which has been and will be the cornerstone of the diagnostic process. The diagnostic
clinician today is able to obtain a seemingly endless amount of information to assess the patient’s oral health, which in turn gives them
and the patient’s other healthcare providers tremendous knowledge about the patient’s overall health and wellness. An excellent
example is the advancement of radiographic imaging. In past five decade many advances came in radiology like computed tomography,
Magnetic resonance imaging, ultrasonography, Arthrography, nuclear medicine, cone beam computed tomography establish perfect
diagnosis that help in treatment planning. In some situations object and superior structures are overlapping to each other, in that case
tomography, computed tomography, cone beam computed tomography will help in 3D contraction of the image. Soft tissue diagnosis in
radiograph is quite difficult because less density, in such case magnetic resonance imaging and ultrasonography will help to reach a
conclusion.
Keywords: Xeroradiography, Digital Radiography, Computerized Tomography
1. Introduction
1.1 Xeroradiography
Various methods have been introduced for obtaining
radiographs among which xeroradiography is a method of
imaging which uses the Xeroradiographic copying process to
record images produced by diagnostic X-rays. It differs from
halide film technique in that it involves neither wet chemical
processing nor the use of dark room.1
Over the past 40 years, Xerox 125 system became applicable
in medical sciences. A prototype Xeroradiographic imaging
system specific for intraoral use was later developed2.
Following these clinical trials showed that xeroradiography is
superior for imaging of dental structures necessary for
successful periodontal and endodontic therapy3.
Xeroradiographic radiation of 90% of more than that for
silver halide radiograph has been reported, while others
found that Xeroradiographic radiation is one third to half of
that for halide radiograph.4
The imaging method was discovered by an American
physicist, Chestor Carlson in 1937.Later, the Xerox company
followed the laboratory investigations of the technique and
its potential applications in medical sciences.Others like
Binnie et al Grant et al and White et al worked on phantoms
and cadavers using the Xerox 125system. Xeroradiography
may be new in dentistry, but in medicine, it had long been
used in the diagnosis of breast diseases, imaging of the larynx
and respiratory tract for foreign bodies, Temporo-mandibular
joint, skull and para-osseus soft tissues.5
Pogorzelska–Stronczak became the first to use
xeroradiography ,to produce dental images while Xerox 125
medical system got adapted for extra oral dental use in
cephalometry, Sialography and panoramic xeroradiography.
Later, a prototype Xeroradiographic imaging system, specific
for intraoral use, was acclaimed to be superior over halide
based intraoral technique.4
Xeroradiography is an electrostatic process which uses an
amorphous selenium photoconductor material, vacuum
deposited on an aluminium substrate to form a plate .the plate
enclosed in tight cassette ,may be linked to films used in
halide based intraoral technique6
The key functional steps in the process involve the
sensitization of the photoconductor plate in the charging
station by depositing a uniform positive charge on its surface
with a corona emitting device called scorotron. That is the
uniform electrostatic charge placed on a layer of selenium is
in electrical contact with a grounded conductive backing .In
the absence of electromagnetic radiation ,the photoconductor
remains non conductive uniform electrostatic charge when
radiation is passed through an object which will vary the
intensity of radiation, observed by Rawls and Owen. The
photoconductor will then conduct its electrostatic charge into
its grounded base in proportion to the intensity of the
exposure. After charging the cassette is inserted into a thin
polyethylene bag to protect the cassette and plate from saliva.
Image Development
The generated latent image is developed through an electro-
phoretic development process using liquid toner. The process
involves the migration to and subsequent deposition of toner
particles suspended in a liquid onto an image reception under
the influence of electrostatic field forces. That is by applying
negatively charged powder (toner) which is attracted to the
residual positive charge pattern on the photoconductor, the
latent image is made visible and the image can be transferred
Paper ID: SUB156149 223
International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
Volume 4 Issue 7, July 2015
www.ijsr.net Licensed Under Creative Commons Attribution CC BY
to a transparent plastic sheet or to a paper .The toner is
thereafter fixed to a receiver sheet onto which a permanent
record is made. The plate is then cleaned of toner for reuse.7
The Xeroradiographic Plate
The plate is made up of a 9 1/2 by 14 inch sheet of
aluminium, a thin layer of vitreous or amorphous selenium
photoconductor, an interface layer, and an over cutting on the
thin selenium layer
The Aluminium Substrate
The substrate for the selenium photoconductor should present
a clean and smooth surface. Surface defects affect the
Xeroradiographic plate’s sensitivity by giving rise to changes
in the electrostatic charge in the photoconductor29
The Interface Layer
This is a thin layer of aluminium oxide between the selenium
photoconductor and aluminium substrate .The oxide is
produced by heat treating the aluminium substrate. As a non
conductor, the interface layer prevents charge exchange
between the substrate and the photoconductor surface.4
The Selenium Coating
The thickness of this layer varies from 150micron meter for
powder toner development .Amorphous or vitreous selenium
coating, is formed by depositing a vapour form of liquefied
selenium in a high vacuum. Because of its ease of use,
fabrication and durability, inherent property of electrical
conduction when exposed to x-rays and ability to insulate
well when shielded from all sources of light, make selenium a
Xeroradiographic material of choice. On the other hand, any
form of impurity adversely affects its performance
.Amorphous form is used in Xeroradiography because
crystalline selenium’s electrical conductivity is very high
which makes it unsuitable in Xeroradiography. However,
amorphous selenium undergoes a dark decay of about 5% per
minute. A new system of xeroradiography which uses plates
with thicker selenium layer (320 micron meter) gives about
50% x-ray absorption.4
Selenium Protective Coating
The protective coating is a 0.1micron meter cellulose acetate
overcoat .The coat bonds intimately with selenium
photoconductor. It helps to prevent degradation of
electrostatic lateral image through the prevention of lateral
conduction of electrostatic charges. Also it impacts positively
on the shelf life of the Xeroradiographic plate.4
Advantages
Elimination of accidental film exposure, High resolution,
Simultaneous evaluation of multiple tissues, Ease of
reviewing, Economic benefit, Reduced exposure to radiation
hazards, Wide applications- Generally Xeroradiography have
interesting application in the management of neoplasm of
laryngo-pharyngeal area, mammary and joint region, as well
as aid in cephalometrics analysis.
Disadvantages
Technical difficulties, Fragile selenium coat, Transient image
retention, Slower speed, Technical limitations
Uses
Diagnosis of periapical pathology in endodontics ,
Assessment of bone height in periodontics.
2. Digital Radiography
Since the discovery of x-rays in 1985, film has been the
primary medium of capturing, displaying and storing
radiographic images. It is a technology that dental
practitioners are the most familiar and comfortable with the
terms of technique and interpretation. Digital radiography is
the latest advancement in dental imaging and is slowly being
adopted by the dental profession .Digital or electronic
imaging has been available for more than a decade. The first
direct digital imaging system, RADIOVISIOGRAPHY
(RVG) was invented by DR FRANCES MOUYANS and
manufactured by TROPHY RADIOLOGIE in 1984.6
Film based and digital imaging principles:
Film based imaging consists of X-ray interaction with
electrons in the film emulsion, production of latent image and
chemical processing that transforms the latent image into
visible one. As such radiographic film provides a medium for
recording displaying and storing diagnostic information .Film
based images are described as analog images. Analog images
are characterized by continuous shades of grey from one area
to the next between the extremes of black and white. Each
shades of grey has an optical density (darkness) related to the
amount of light that can pass through the image at a specific
site. Film displays higher resolution than digital receptors
with a resolving power of about 161p/mm. However, film is
relatively inefficient detector and thus, requires relatively
high radiation exposure. The use of rectangular collimation
and the highest film speeds are the method that reduces
radiation exposure; however the radiation exposure is still 2
to 3 times more than digital radiographic technique. The
other disadvantages include chemicals to process the image
and are often the source of errors and retakes.7
The final
result is a fixed image that is difficult to manipulate once
captured .Digital imaging is the result of X-rays interaction
with electrons in electronic sensor pixels (picture elements),
conversion of analog data to a digital data, computer
processing and display of the visible image on a computer
screen. Data acquired by the sensor is communicated to the
computer in analog form. Direct digital imaging systems
produce dynamic images that permit immediate display,
image enhancement storage, retrieval and transmission.
Digital sensors are more sensitive than film and require
significantly lower radiation exposure. It is a method of
capturing a radiographic image using a sensor breaking it
into an electronic pieces and presenting and storing an image
using a computer. The sensor is used to receive the analog
information and through analog to digital converter (ADC) to
convert it to a digital image, array of picture elements called
pixels, with discrete great values for each one. Special
software is used to store and manipulate the digital image in
the computer.7
Two digital radiography systems rely on the sensor-the direct
and the indirect methods. A number of components are
required for direct digital image production. These
components include an X-ray source an electronic sensor, a
Paper ID: SUB156149 224
International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
Volume 4 Issue 7, July 2015
www.ijsr.net Licensed Under Creative Commons Attribution CC BY
digital interface card, a computer with an analog to digital
converter (ADC), screen monitor software and a printer.
Direct digital sensors are either a charged coupled device
(CCD) or a complementary metal oxide semiconductor active
pixel sensor (CMOS-APS).The CCD is a solid state detector
composed of an array of x-ray on a light or a light sensitive
pixels on a pure silicon chip. A pixel or a picture element
consists of a small electron well into which the x-ray light
energy is deposited upon exposure. The individual CCD
pixel size is approximately 40micron with the latest version
in the 20 micron range. The rows of pixels are arranged in a
matrix of 1604 x 1208 pixels which is increased up to 2048
X 1680 pixels.8 Charge coupling is a process whereby the
number of electrons deposited in each pixel are transferred
from one well to the next in a sequential manner to a read-
out amplifier for image display on a monitor. There are two
types of digital sensor array designs area and linear. Area
arrays are used for intraoral radiography while linear arrays
are used for extra oral imaging. Area arrays are available in
sizes comparable to size zero, size 1and size 2 films, but the
sensors are rigid and thicker than radiographic film and have
a smaller sensitive area for image capture. The sensor
communicates with a computer through an electric cable.
Area array CCD has two primary formats: Fibre-optically
coupled sensors and direct sensors. Fibre-optically coupled
sensors utilize a scintillation screen coupled to a CCD. When
x-ray interacts with a screen material, light photons are
generated, detected and stored by CCD .Direct sensor CCD
arrays capture the image directly. The complementary metal
oxide semiconductor active pixel sensor (CMOS-APS) is the
latest development in direct digital sensor technology.
Externally CMOS sensors appear identical to CCD detectors
but they use an active pixel technology and are less
expensive to manufacturers
Advantages of Direct Digital Imaging
While digital X-ray sensors have long equalled analog film
for diagnostic tasks, they have several advantages over film
radiography, including the following9:-
Immediate image production with solid state device such as a
charged coupled device (CCD) and a complementary metal
oxide semiconductor (CMOS), Interactive display on a
monitor with the ability to enhance image features and make
direct measurements, Integrated storage, providing access to
images through practice management software system,
Security of available backup and offside anchoring, Perfect
image duplicates to accompany referrals to other referrals to
other practitioners, Security mechanisms to identify original
images and differentiate them from altered images, Ability to
tag information such as patient identifier, date of exposure
and other relevant details, Interloper ability of the digital
imaging and communications in Medicines (DICOM) which
enables Clinicians with difficult equipment and software to
view and the same image, The actual amount of exposure
reduction is dependent on a number of factors including film
speed sensor area, collimation and retakes
Disadvantages of Direct Digital Imaging
Rigidity and thickness of the sensor, Decreased resolution,
Higher initial system cost, Unknown sensor lifespan transfer,
Perfect semiconductor charge transfer, Infection control,
Legal issues, Sterilization.
3. Indirect or Scanned Digital Imaging
Technique
As with direct systems, there are advantages and
disadvantages to indirect systems. The smaller size and lack
of a cord may make intraoral placement of phosphor plates
easier than placement of direct sensors. Phosphorplates are
somewhat flexible, but the corners cannot be bent (as is
sometimes done with film) without damaging the plates.
Phosphor plates can potentially be reused hundreds of times,
but are susceptible to scratching which will shorten their
useful life. Phosphor plates are light sensitive and exposure
to ambient light must be minimized during the time period
between removal from their protective cover and placement
into the scanner. The length of time that plates are exposed to
ambient light during this transfer process will determine the
level of allowable ambient light at the scanner location.
Scanners in which plates are loaded directly into a slot can
generally be used in areas of higher ambient light compared
to systems in which the plates are loaded ondrums prior to
placement in the scanner.
The primary disadvantage of phosphor plate systems involves
the time required to scan and erase the plates. Following
exposure, plates must be removed from their contaminated
barrier pouches, run through the scanner, "erased" with bright
light, and repackaged in clean barrier pouches prior to using
again. For plate erasure, some scanners incorporate an
"erase" cycle within the scanner itself. With other units the
plates are moved to a separate plate eraser following the
scanning process. It is less expensive to purchase enough
phosphor plates to place in every operatory compared to
purchasing enough direct sensors for every operatory. The
cost for an intraoral phosphor plate is less than twenty-five
dollars compared to several thousand dollars for each direct
sensor. Most existing panoramic and cephalometric units do
not require expensive up grades for use with phosphor plates.
Phosphor plates are simply placed in cassettes similar to film,
except that no intensifying screens are used. Therefore, the
same panoramic or cephalometric unit can be used to expose
either conventional film or phosphor plates. Phosphor plate
systems require purchase of a scanner.
Imaging area dimensions
The imaging area of direct sensors and phosphor plates can
be compared to the imaging area of conventional dental film.
Listed here are conventional film sizes for comparison
purposes.
Size 0: 22 x 35 mm
Size 1: 24 x 40 mm
Size 2: 31 x 41 mm
Size 3: 27 x 54 mm
Size 4: 57 x 76 mm
Paper ID: SUB156149 225
International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
Volume 4 Issue 7, July 2015
www.ijsr.net Licensed Under Creative Commons Attribution CC BY
The essential components of indirect digital imaging system
include a CCD camera and computer. In this method the
existing x-ray film is digitalized using a CCD camera. The
CCD camera scans the image and then displays it on the
computer monitor. This concept is similar in theory to
scanning and image such as a photograph to a computer
screen
Advantage of Indirect Digital Imaging
Thin sensor compared to direct digital imaging so the patient
is more comfortable.
Disadvantage of Indirect Digital Imaging
Indirect digital imaging is inferior to direct digital imaging
because the resultant image is similar to a “COPY” of the
image versus the original.
4. Computed Tomography
History
The development of computed tomography was a milestone
in radiology. Mathematician J.Radon laid the theoretical
foundation for its development in 1917 when he
demonstrated that the image of an object could be produced
from a infinite sets of all of its projections. This foundation
was first applied to Radio astronomy, optics and electron
neuroscopy. This first clinical computed tomography X-ray
unit first developed in 1972 by G.N. Horse field in England10
The pursuit of 3-d information has led to exploring the value
of CT for the assessment of alveolar bone height. While CT
provides exquisite 3D views, its ability to show very small
details remains limited , usually not more than 1-2 mm.
Although the level of image remains considerable lower than
with conventional intraoral imaging, these advancements in
CT technology satisfy almost all periodontal imaging needs
from a pure technical and possibly diagnostic perspective.
Studies have shown that CT assessment of alveolar bone
height and intrabony pockets is reasonably accurate and
precise11
Technique
The technique produces an axial cross sectional image of the
head using a narrowly collimated, moving beam of X-rays. A
scintillation crystal detected the remnant radiation of this
beam, and the resulting analog signal was fed into a
computer, digitalized analyzed by a mathematical algorithm
and the data reconstructed as an axial tomographic image.
The image produced by this technique was like no other x-
ray image claimed to be 100 times more sensitive than
conventional x-ray systems, it demonstrated differences
between various soft tissues never before seen with x-ray
imaging techniques. Since1972 computed tomography has
many names, each of which referred to at least one aspect of
the technique- computerized, axial tomography computerized
reconstruction tomography, computed tomography scanning,
axial tomography and computerized trans axial tomography.
Currently the preferred name is COMPUTED
TOMOGRAPHY, abbreviated at CT.12
A CT scanner consists of a radiographic tube that emits
finely collimated, fan shaped X-ray beam directed to a series
of scintillation detectors or ionization chambers. Depending
on the scanner mechanical geometry both the radiographic
tube and detectors may rotate synchronously about the
patient and the x ray tube may move in a circle within the
detector ring. CT scanners that employ this type of movement
for image acquisition are called INCREMENTAL
SCANNERS because the final image set consist of a series of
continuous or overlapping axial images.
5. Newer Development
More recently CT scanner has been developed that acquire
image data in a SPIRAL OR HELICAL fashion. With these
scanners while the gantry containing the x ray tube and
detectors revolves around the patient, the table on which the
patient is lying continuously advances through the gantry.
This results in the acquisition of continuous spiral of data as
the x ray beam moves down the patient. It is reported that
compared incremental CT scanner spiral scanners provide
multiplanner image construction reduce radiation dose (upto
75%). Regardless of the mechanical geometry, the
transmission signal recorded by the detectors represents a
composite of the absorption characteristics of all elements of
the patient in the path of x ray beam.13
The CT image is a digital image reconstructed by computer,
which mathematically manipulates the transmission data
obtained from multiple projections. For example if one
projection is made every one third of a degree, 1080
projections results during the course of a single 360 degree
rotation of the scanner about the patient. Data derived from
these 1080 projections (1080 projection constitute one scan)
contain all the information necessary to construct a single
image. The CT image is recorded and displayed as a matrix
of individual blocks called VOXELS (volume elements).
Each square of image matrix is PIXEL whereas the size of
the pixel ( about 0.1mm ) is determined partly by the
computer program used to construct the image, the length of
the voxel (about 1- 20 mm) is determined by the width of x
ray beam, which in turn is controlled by the pre-patient and
post-patient collimators. Voxel length is analogues to the
tomographic layer in film topography. For image display,
each pixel is arranged a CT number representing dentistry.
This number is proportional to the degree to which the
material within the voxel has attenuated the X-ray beam. It
represents the absorption characteristics or linear alteration
coefficient of that particular volume of tissue in the patient
CT numbers also known as HOUSEFIELD UNITS, may
range from -1000 to +1000, each constitute the different level
of optical dentistry. This scale of relative densities is based
on air (-1000), water (0) and dense bone (+1000).
Paper ID: SUB156149 226
International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
Volume 4 Issue 7, July 2015
www.ijsr.net Licensed Under Creative Commons Attribution CC BY
Advantages
CT has several advantages over conventional film
radiography and film tomography.
CT completely eliminates the superimposition of images of
structures outside the area of interest, Because of the inherent
high contrast resolution of CT, differences between tissues
that differ in physical density by less than 1% can be
distinguished. Conventional radiography requires 10%
difference in physical density to distinguish between tissues.,
Data from a single CT imaging procedure consisting of either
multiple contiguous or one helical scan can be viewed as
images, in the axial, coronal or saggital planes depending on
the diagnostic task. This is referred to as MUTIPLANER
REFORMATTED IMAGING, CT scan is superior to MRI
when evaluating skull fractures, CT scan can provide detailed
images of brain nervous system, CT is excellent for
individuals involved in head trauma, CT is much cheaper
than MRI and equally as fast, Artefacts are less of problem
with a CT scan compared to MRI.14
Disadvantages
Increased radiation dosage. (This is about same radiation
exposure that a normal individual will get in about 2 months.,
Economically not used for routine dental treatment, CT scan
should never be done in a pregnant female because of the
exposure of radiation risk to the foetus, The dye used in CT
is iodine based and is often a cause of allergy. The dye can
also lead to kidney failure in individuals with diabetes,
Unlike adults CT scan should not be repeated in children
because of the repeated radiation exposure, CT is not very
good at identifying pathosis of soft tissue, CT is not good at
identifying areas of inflammation or infection of the brain.
Uses
Primarily because of its high contrast resolution and ability to
demonstrate small differences in soft tissue density, CT has
become useful for the diagnosis of disease in the
maxillofacial complex, including the salivary gland and TMJ.
However with the advent of magnetic resonance imaging,
which has proved superior to CT for depicting soft tissues the
use of CT scanning for assessment of internal de-arrangement
of the TMJ has decreased significantly. Additionally CT has
been shown to be useful for evaluation of patients before
placement of endo-osseous oral implants.
Despite the fact the similar information about the maxillary
and mandibular anatomy can be obtained with film
tomography, CT allows reconstruction of cross section image
of the entire maxilla or mandible or both from a single
imaging procedure. One of the first applications of 3D CT
was the study of patient with suspected intervertebral disc
herniation and spinal stenosis. The 3D CT has been applied
to craniofacial reconstructive surgery and has been used both
for congenital and acquired deformities and for evaluation of
intracranial tumours, benign and malignant lesions of the
maxillofacial region, cranial spine injuries pelvic fracture,
and deformities of hand and feet. The availability of data in
three dimension format also has allowed the construction of
life sized model that can be used for trial surgeries and the
construction of surgical stents for guarding dental implants
placement, as well as the creation of accurate implanted
prosthesis.
CT technology is been continually advanced. I matron ( San
Fransisco) has developed a CT scanners capable of acquiring
data up to 10 times faster than conventional CT. Its ultrafast
CT, which has scan times on the order of 50msec, is able to
freeze cardiac and pulp monary motion enhancing the quality
without motion artefacts
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